The Mezzanine

If it's just for 12 days I would be keeping them completely separate. Cats are extremely territorial and it can take months to integrate them properly - if you rush it you're likely to have injuries from fighting (ie, the kind of injuries that cost money at the vet!) as well as stress-peeing in places that you might not find for a while. I'd be confining Prince to the spare room with a bed, toys, food and litter tray, then letting him out for an hour or so in the evening for a run around your flat while Elf is confined to your room.